In recent years, a planar light-emitting body to be used as backlight of various displays, display panels of sign boards and emergency light, lighting devices and the like has been drawing attention because it has many excellent characteristics such as high luminance, high efficiency, thin type and lightness in weight. As one of the planar light-emitting bodies, there is an organic electroluminescence element (hereinafter, referred to as organic EL element), which emits light by supplying electric energy to a light-emitting layer formed of an organic material from respective positive and negative electrodes. The organic EL element particularly draws attention for the reason that it can emit light with a low voltage of about several volts to several ten volts, and that space saving can be achieved because it is a thin film-type complete solid element.
The organic EL element includes, generally, a transparent substrate, a first electrode, a light-emitting layer and a second electrode, and the first electrode, the light-emitting layer and the second electrode are formed on the transparent substrate in this order. In the organic EL element of the configuration, when a voltage is applied between the first electrode and the second electrode, holes are injected into the light-emitting layer from one electrode and electrons are injected into the light-emitting layer from the other electrode. Subsequently, holes and electrons injected in the light-emitting layer recombine in the light-emitting layer, and thus light is emitted. Then, light emitted from the light-emitting layer (hereinafter, referred to as emitted light) passes through the first electrode and the transparent substrate, and a user recognizes that the organic EL element emits light. Meanwhile, the light-emitting layer includes one or a plurality of organic compound layers containing an organic light-emitting substance (light-emitting material), and depending on the kind of the organic light-emitting substance to be added, the wavelength of the emitted light can be varied.
Meanwhile, when the planar light-emitting body is used as a light source of a lighting device or the like, extraction of white light is required. In the organic EL element, as techniques for obtaining white light, there are techniques to stack a plurality of light-emitting layers each containing a light-emitting material that emits light having wavelengths different from each other, and techniques to mix a plurality of light-emitting materials each emits light having wavelengths different from each other in one light-emitting layer. When such a technique is used, emitted light from each light-emitting material is mixed and white light is obtained. Meanwhile, in the technique, by changing suitably the kind of the light-emitting material to be used, light of various colors can be extracted, in addition to white light.
However, in the case of a planar light-emitting element constituted by a thin film such as an organic EL element, the light emitted from the light-emitting layer does not have directivity. Therefore, among the emitted light, a light component having an emission angle of a critical angle or more, of the light, which is determined by the refractive index of a thin-film layer including the light-emitting layer and the refractive index of a medium through which the emitted light passes at the time of light emission, is totally reflected at the interface of the medium and is confined inside the organic EL element and serves as guided light that propagates through the inside. That is, there occurs a problem in which light components having emission angles of less than the critical angle can be extracted outside among the light generated in the light-emitting layer, but that other light components are lost to lower a light extraction efficiency (a ratio of the energy of light emitted outside through the transparent substrate relative to the energy of the emitted light).
Here, the above-mentioned problem of the light extraction efficiency will be explained more specifically. When the light extraction efficiency (light-emitting efficiency) was derived by applying the analysis of a multiple reflection phenomenon based on the classic optics, to an organic EL element in which the refractive index of a light-emitting layer is n, the light extraction efficiency can be approximated by 1/(2n2), and is almost determined by the refractive index n of the light-emitting layer. When calculating simply the light extraction efficiency of the organic EL element on the basis of the approximation, assuming that the refractive index n of the light-emitting layer is about 1.7, the light extraction efficiency of the organic EL element is about 20%. Therefore, remaining light of about 80% propagates into the direction inside the plane of the light-emitting layer (disappearance in the lateral direction), or disappears at a metal electrode (cathode) provided in a position facing the transparent electrode (anode) with the light-emitting layer interposed (absorption backward). That is, in organic EL elements, among light generated inside the layer (the refractive index is about 1.7 to 2.1) having a refractive index higher than that of the air, light of only about 15% to 20% can be extracted outside.
Therefore, conventionally, there are proposed various technologies to improve the light extraction efficiency in organic EL elements. Specifically, there are proposed techniques such as introducing, between a substrate and a light-emitting body, a flat layer having a refractive index that lies midway between refractive indices of both, to thereby achieve antireflection, and forming a light diffusion layer containing a binder and a filler on the surface of the substrate, to thereby inhibit the total reflection of the emitted light through the utilization of the scattering effect of the light diffusion layer. Furthermore, conventionally, there is also proposed a technique of inhibiting the total reflection at the interface between the transparent substrate and the air by forming a prism sheet or a microarray lens on the surface of a transparent substrate to thereby provide a concave-convex shape such as a prism shape or a lens shape on the surface of the transparent substrate (for example, see Patent Literature 1). Moreover, conventionally, there is also proposed a technique of improving the light-emitting efficiency itself of the light-emitting layer by devising the configuration of the light-emitting layer (such as making the thickness of the light-emitting layer larger).